If you think back to "Rightful Heir", the sword of Kahless was in the monestary on Boreth. Kahless himself picked it up and claimed that it was his sword, and everybody agreed. So how come it's now been missing for hundreds of years?

Plotline

Kor asks Dax and Worf to accompany him on a mission to recover the fabled Sword of Khaless, one of the most significant of Klingon historical artifacts. They are successful, but jealousy soon sets in and the party are soon at one anothers throats.